This invention relates to processing of semiconductor wafers in a vacuum chamber and, more particularly, to methods and apparatus for gas-assisted thermal transfer utilizing differential vacuum pumping to reduce gas leakage into the vacuum chamber.
In the fabrication of integrated circuits, a number of processes have been established which involve the application of high energy beams onto semiconductor wafers in vacuum. These processes include ion implantation, ion beam milling and reactive ion etching. In each instance, a beam of ions is generated in a source and directed with varying degrees of acceleration toward a target. Ion implantation has become a standard technique for introducing impurities into semiconductor wafers. Impurities are introduced into the bulk of semiconductor wafers by using the momentum of energetic ions as a means of imbedding them in the crystalline lattice of the semiconductor material.
As energetic ions impinge on a semiconductor wafer and travel into the bulk, heat is generated by the atomic collisions. This heat can become significant as the energy level or current level of the ion beam is increased and can result in uncontrolled diffusion of impurities beyond prescribed limits. A more severe problem with heating is the degradation of patterned photoresist layers which are applied to semiconductor wafers before processing and which have relatively low melting points.
In commercial semiconductor processing, a major objective is to achieve a high throughput in terms of wafers processed per unit time. One way to achieve high throughput in an ion beam system is to use a relatively high power beam. Large amounts of heat may be generated. Thus, it is necessary to provide cooling of the wafer in order to prevent elevated temperatures from being attained.
Techniques for keeping the wafer temperature below a prescribed limit have included batch processing in which the incident power is spread over a large area, time-shared scanning of the beam, and conductive cooling through direct solid-to-solid contact between a wafer and a heat sink (pending application Ser. No. 284,915, filed July 20, 1981; and U.S. Pat. No. 4,282,924, issued Aug. 11, 1981, to Faretra). The cooling efficiency of systems employing solid-to-solid contact is limited by the extent to which the backside of the wafer contacts the thermally conductive surface, since, at the microscopic level, only small areas of the two surfaces (typically less than 5%) actually come into contact.
The technique of gas conduction is known to permit thermal coupling between two opposed surfaces and has been widely employed. In U.S. Pat. No. 3,566,960, "Cooling Apparatus For Vacuum Chamber," Stewart, the problem of inadequate contact between solid surfaces is discussed; and a circulating gaseous or liquid medium to cool the workpiece in the vacuum chamber is disclosed. In the same vein, gas conduction cooling of a workpiece, preferably a semiconductor wafer in a vacuum, is shown in King et al, "Experiments on Gas Cooling of Wafers," Proc. 3rd Inter. Conf. on Ion Implantation Equipment and Techniques, Queens University, Kingston, Ontario, May 1980, and in U.S. Pat. No. 4,264,762, "Method Of Conducting Heat To Or From An Article Being Treated Under Vacuum," King. In this apparatus, gas is introduced into the middle of a cavity behind a semiconductor wafer. Thermal coupling between a support plate and the wafer is achieved through a gas as typically accomplished in the gas conduction art.
Gas-assisted, solid-to-solid thermal transfer with semiconductor wafer is disclosed in pending application Ser. No. 381,669, filed May 25, 1982, and assigned to the assignee of the present application. A semiconductor wafer is clamped at its periphery onto a shaped platen. Gas under pressure is introduced into the microscopic void region between the platen and the wafer. The gas presssure approaches that of the preloading clamping pressure without any appreciable increase in the wafer-to-platen spacing. Since the gas pressure is significantly increased without any increase in the wafer-to-platen gap, the thermal resistance is reduced, and solid-to-solid thermal transfer with gas assistance produces optimum results.
When gas is utilized to transfer heat between a wafer and a heat sink during vacuum processing, it is necessary to confine the gas to the region behind the wafer and to prevent escape of the gas into the vacuum chamber. Gas escaping into the vacuum chamber is likely to have deleterious effects on the process being performed. For example, ion implantation requires pressures on the order of 10.sup.-6 Torr in order to implant highly accurate, highly uniform impurity doses. When the pressure in the vacuum chamber increases due to gas leakage, the ion beam experiences neutralizing collisions with gas molecules. As a result, the dose measurement accuracy, which relies upon charge measurement, is reduced.
Typically, elastomer O-rings have been utilized to isolate the region containing the thermal transfer gas from the vacuum chamber. In U.S. Pat. No. 4,264,762 and in pending application Ser. No. 381,669, an O-ring is positioned between the backside of the wafer and the platen near the wafer periphery. While this arrangement provides generally satisfactory sealing, it has certain drawbacks. Due to the elevated temperatures, the wafer can stick to the O-ring, thereby causing an interruption in processing. Furthermore, the O-ring must be located substantially inward of the wafer edge to avoid the wafer flat used for crystal orientation. Therefore, the outer edge portion of the wafer is not cooled. If the wafer is clamped outside the O-ring radius, undesired stresses are applied to the wafer. W. N. Hammer in "Cooling Ion Implantation Target," IBM Technical Disclosure Bulletin, Vol. 19, No. 6, November 1976, discloses apparatus in which gas flows through a chamber behind the wafer. The gas-filled chamber is sealed from the vacuum chamber by an O-ring around the periphery of the front surface of the wafer. The potential for sticking of the front surface of the wafer to the O-ring is even more severe, since photoresist is usually applied to the front surface of the wafer. Furthermore, there exists the possibility of wafer contamination by the material of the O-ring. As described above, the O-ring must be located inwardly of the outer edge of the wafer due to the wafer flat. It is, therefore, desirable to provide a system in which gas leakage between the backside of the wafer and the vacuum chamber is minimized and in which the problems described above are alleviated.
It is a general object of the present invention to provide novel methods and apparatus for gas-assisted thermal transfer with a semiconductor wafer.
It is another object of the present invention to provide methods and apparatus for reducing the leakage into a vacuum processing chamber of a thermal transfer gas introduced behind a semiconductor wafer.
It is yet another object of the present invention to provide methods and apparatus for differential pumping of a thermal transfer gas during vacuum processing of a semiconductor wafer.
It is still another object of the present invention to provide methods and apparatus for gas-assisted thermal transfer with a semiconductor wafer, wherein elastomer sealing materials are not required to be in contact with the wafer.